drm/msm/dpu: Extract plane splitting into a dedicated function

drm/msm/dpu: Extract plane splitting into a dedicated function

dpu_plane_atomic_check_nosspp() currently handles both plane
validation and plane splitting. For better simplicity and to
facilitate future refactoring, move the splitting logic into
its own dedicated function.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Jun Nie <jun.nie@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/711319/
Link: https://lore.kernel.org/r/20260312-msm-next-quad-pipe-split-v19-1-4ffa2b06c996@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>